Add local login, agent extra log paths, IPv4/IPv6 metrics; remediate security audit findings

This is a large squashed commit covering two batches of prior uncommitted
work plus a full security-audit remediation pass, kept together because
go.mod/go.sum and several shared files (main.go, handler.go) were touched
by both and splitting risked non-building intermediate commits.

Features (built earlier, previously uncommitted):
- Local username/password login for single-tenant deployments with no
  SSO configured (api/localauth, alerting/internal/sessioncheck,
  sentryctl users, web/src/routes/login, metadata migrations 0040/0041).
- Remotely-editable additional log file paths for agents, on top of
  their existing primary source (api/agents, agent/sentry-agent
  extra-file-path diffing, web agent config UI).
- IPv4/IPv6 addresses reported alongside other host system metrics.

Security audit remediation (this pass, all live-verified in production):
- Critical: block ClickHouse SSRF table functions (url/remote/file/s3/...)
  in the raw-SQL query escape hatch.
- High: deny sensitive paths and require Admin to add agent
  extra_file_paths (Editor could previously point an agent at /etc/shadow
  or an SSH key); alerting webhook targets now validate against
  internal/metadata/loopback addresses, both at creation and send time;
  alerting's session middleware now enforces an Editor+ floor on
  mutating requests instead of "any authenticated session"; bumped
  goxmldsig to close a SAML signature-verification bypass (GO-2026-4753).
- Medium: per-IP login rate limiting; security response headers
  (HSTS/CSP/nosniff/X-Frame-Options/Referrer-Policy/Permissions-Policy)
  on web/nginx.conf; a DevCredentialWarnings check in every Go service's
  config loader, logging loudly at startup if a deployment is still on
  docker-compose.yml's literal dev-only credentials; dependency bumps
  (golang.org/x/text, grpc, x/net, quick-xml, h2) across every affected
  Go module and both Rust crates, including a previously-uncovered x/net
  vulnerability in deploy/operator; a new security-scan.yml CI workflow
  running cargo-deny/govulncheck/npm-audit, mirroring the existing
  license-compliance.yml matrix shape.
- Low: removed sentryctl's plaintext --password flag (shell
  history/`ps` exposure) in favor of stdin and a --password-stdin flag
  for reset-password's optional specific-password path; a dummy bcrypt
  comparison closes a login response-time username-enumeration
  side-channel.

agent/README.md

@@ -153,6 +153,30 @@ alerting engine already detects natively via an `absence`-condition
 alert rule. See `/docs/agent-heartbeat-monitoring.md` for the exact rule
 to create.
 
+## Host CPU/memory/disk metrics
+
+Same shape as heartbeat, same reasoning: `[metrics]` in the config
+(`enabled = false` by default) sends a periodic record — CPU%, memory
+used/total, disk used/total for `/` — tagged `sentry.metrics=true`, with
+the individual numbers as their own attributes (`cpu_percent`,
+`mem_used_bytes`, `mem_total_bytes`, `disk_used_bytes`,
+`disk_total_bytes`), queryable directly (e.g. `cpu_percent > 80`) since
+the query language transparently maps any non-standard field name to
+`attributes['field']` with automatic numeric casting. Powers the web
+UI's "Hosts" nav section. Linux-only for now (`src/metrics.rs`) — reads
+`/proc/stat`/`/proc/meminfo` and shells out to `df`, no new
+dependencies, same "shell out to a boring, ubiquitous tool" precedent
+`journalctl` already sets.
+
+**Enable this on only one agent process per physical host.** It's
+common for one host to run several `sentry-agent` processes (one per
+log source, each needing its own `[agent] host` value to work around
+the `agents` table's `UNIQUE (tenant_id, host)` constraint — see
+`/docs/agent-management-design.md`) — turning `[metrics]` on for more
+than one of them reports the same physical machine as multiple
+different "hosts" with conflicting metric series. Pick the one process
+using that host's real, unoverridden hostname.
+
 ## Running as a Windows service
 
 "A native Windows service, not a WSL wrapper" means implementing the Win32